1. Field of the Invention
The invention is generally related to analysis of subterranean formations, and more particularly to measurement of sound speed of a downhole fluid to facilitate acoustic logging operations for formation evaluation and reservoir characterization.
2. Background of the Invention
Wireline and logging-while-drilling (LWD) tools are used to measure physical, chemical, and structural characteristics of formations surrounding a borehole. For example, data gathered by logging tools can be used to interpret formation stratigraphy, lithology, and mineralogy. Generally, the logging tool emits an acoustic, electromagnetic or optical signal and measures the response. In the case of acoustic logging tools, changes in amplitude, phase and speed of the received acoustic energy can be utilized to characterize the formation. Early generation acoustic logging tools extract formation compressional and shear speed from head waves. Consequently, those logging tools do not require knowledge of mud speed. Newer generation acoustic logging tools utilize acoustical modal propagations, such as Stoneley, dipole and quadrupole modes. Sound speed (and its reciprocal, sound slowness) affects the characterization of these modes. In particular, the propagation characteristics of these modes depend on the formation speed, borehole mud speed, and other parameters. Consequently, in order to obtain accurate measurement of the formation speed, it is desirable to obtain an accurate indication of mud sound speed independently.
Techniques for measuring fluid sound speed are known. Outside the borehole environment a wide variety of sound speed measurement equipment is available. However, the results obtained by measuring mud sound speed by examining mud at the surface are relatively inaccurate because sound speed of borehole mud is a function of its constituents, temperature and pressure. Consequently, the speed can vary over depth and time, and would change during transport to the surface.